1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, for reading an image of an original, for example, and forming the image on a sheet in accordance with a signal thus read.
2. Description of the Related Art
A general operation of a copying machine is as follows. That is, a uniform electrostatic charge is applied on the surface of a photosensitive drum and the surface charge on an exposed portion is released by image exposure, so as to form an electrostatic latent image. The latent image is then visualized (developed) by supplying toner charged with a polarity opposite to that of the latent image, and thus obtained toner image is transferred on a sheet. Then, as the sheet is allowed to pass through a fixation device, the toner (image) is fixed on the sheet by heat.
As the above photosensitive drum, an OPC (organic photoconductor) drum is used, and a discharge of a negative polarity is carried out on the photosensitive drum by a charging device (charger). In order to stabilize the discharge characteristics of the charging device, a grid is used.
A high-voltage transformer (high-voltage power source) is assigned exclusively to each of the charging device, the grid and the development roller. The operations of these members are ON/OFF-controlled in an open-loop manner by a CPU serving to control the apparatus as a whole, in accordance with the timing for the formation of an image. Further, in order to stabilize the output to a preset value, each of the high-voltage transformers carries out a feedback operation.
In the above-described structure, the high-voltage transformers are independent from each other, and therefore the ON/OFF control, the output adjustment and the like are separately carried out.
Needless to say, an appropriate image is formed only when an output is output as it is supposed to be. However, in some cases, an abnormal output is generated, resulting in spending an excessive amount of toner, or causing damage to the apparatus such as a carrier developer. The abnormality discussed here includes, not only an abnormal output of the high-voltage transformers, but also abnormal operations due to an erroneous setting of the charging device or the disconnection of the charging wire (discharge wire). Further, if a grid voltage and a development bias voltage are supplied not at different timings with a predetermined interval therebetween, but at the same timing, the entire surface of the drum is blacked regardless of the image of an original to be copied, i.e. a so-called blacking development occurs, at the start of the image formation. In the case where the supply of the grid voltage and the supply of the development bias voltage are stopped at the same time, the charged surface of the drum exhibits the carrier development while passing the development roller.
Conventionally, in order to solve the above-described drawback, a circuit for outputting a signal indicating an abnormality to the CPU serving as a controller when an output from the high-voltage transformers becomes abnormal, or a switch for detecting the setting of the charging device is provided so as to control the output from the transformers when the CPU detects an abnormal signal or insufficient setting of the device, thus avoiding an abnormal image. However, with such a copying apparatus, the circuit structures of the high-voltage transformers and the apparatus itself are rendered complex, and accordingly, the production cost is inevitably increased.